Some subterranean deposits of viscous petroleum can be extracted in situ by lowering the viscosity of the petroleum to mobilize it so that it can be moved to, and recovered from, a production well. Reservoirs of such deposits may be referred to as reservoirs of heavy hydrocarbon, heavy oil, bitumen, tar sands, or oil sands. The in situ processes for recovering oil from oil sands typically involve the use of multiple wells drilled into the reservoir, and are assisted or aided by injecting a heated fluid such as steam into the reservoir formation from an injection well.
For example, one process for recovering viscous hydrocarbons is known as steam-assisted gravity drainage (SAGD). A typical SAGD process utilizes one or more pairs of vertically spaced horizontal wells. Various embodiments of the SAGD process are described in Canadian Patent No. 1,304,287 and corresponding U.S. Pat. No. 4,344,485. In a SAGD process, steam is pumped through an upper, horizontal, injection well into a viscous hydrocarbon reservoir while hydrocarbons are produced from a lower, parallel, horizontal, production well vertically spaced proximate to the injection well. The injection and production wells are typically located near, but some distance above, the bottom of the primary pay zone in the hydrocarbon deposit. In a SAGD process, the injected steam initially mobilises the in-place hydrocarbons to create a “steam chamber” in the reservoir around and above the horizontal injection well. The term “steam chamber” means the volume of the reservoir which is saturated with injected steam and from which mobilised oil has at least partially drained. As the steam chamber expands upwardly and laterally from the injection well, viscous hydrocarbons in the reservoir are heated and mobilised, especially at the margins of the steam chamber where the steam condenses and heats a layer of viscous hydrocarbons by thermal conduction. The mobilised hydrocarbons (and aqueous condensate) drain under the effects of gravity towards the bottom of the steam chamber, where the production well is located. The mobilised hydrocarbons are collected and produced from the production well. The rate of steam injection and the rate of hydrocarbon production may be modulated to control the growth of the steam chamber to ensure that the production well remains located at the bottom of the steam chamber in an appropriate position to collect mobilised hydrocarbons.
Alternative recovery processes may employ thermal and non-thermal components to mobilise oil. For example, light hydrocarbons may be used to mobilise heavy oil. U.S. Pat. No. 5,407,009 teaches an exemplary technique of injecting a hydrocarbon solvent vapour, such as ethane, propane or butane, to mobilise hydrocarbons in the reservoir.
While it is common to position the production wells at the lower portion of the reservoir to maximize the economic benefit of the recovery operation, the horizontal sections of the production wells are normally positioned about 2 m or more above the base due to practical and economic considerations. For instance, it is generally considered good practice when drilling production wells to limit the inclination of the well to within 5 m. That is, the elevation difference between the toe of the well and the heel of the well is less than about 5 m. It is also considered good practice in a SAGD setup to ensure that no portion of the production well is higher than any portion of the injection well. Thus, the inclination of the wells is also limited by the vertical separation between the injection well and the production well, which is typically about 5 m. As is commonly known, the depth in a reservoir can be expressed as the measured depth (MD), the true vertical depth (TVD), or the TVD subsea (TVDSS). A known technique for completing a perforated portion of a well is the tubing-conveyed perforating (TCP) completion technique. A common tool for measuring oil saturation (typically denoted as So) in the reservoir is known as the Reservoir Saturation Tool (RST).